Devices employing liquid crystals have found use in a variety of electrooptical applications, in particular those which require compact, energy-efficient, voltage-controlled light valves, such as watch and calculator displays, and flat-panel displays as are found in portable computers and compact televisions.
Liquid crystal displays have a number of unique characteristics, including low voltage and low power of operation, which make them the most promising of the non-emissive electrooptical display candidates currently available. However, slow response and insufficient nonlinearity can impose limitations for many potential applications. The requirement for speed may become especially important in proportion to the number of elements which have to be addressed in a device. This limits the potential use of some types of liquid crystals.
The modes of liquid crystal displays that are most extensively employed at the present are twisted nematic (TN), supertwisted birefringence effect (SBE), and dynamic scattering (DS), all employing nematic or chiral nematic (cholesteric) liquid crystals. These devices are based upon the dielectric alignment effects (Freedericksz effect) of the nematic and/or chiral nematic liquid crystal or mixtures of nematic or chiral nematic liquid crystals upon application of an electric field. The average molecular long axis of the liquid crystal material takes up a preferred orientation in the applied electric field, the orientation of which is dependent on the sign of the dielectric anisotropy of the material or mixture, and this orientation relaxes upon removal of the applied electric field. This reorientation and relaxation is slow, on the order of a few milliseconds.
Although nematic and chiral nematic liquid crystals are the most extensively employed, there are liquid crystal devices that employ higher ordered smectic liquid crystals.
Devices employing materials with a smectic A mesophase are useful in device applications as described in Crossland, et al. U.S. Pat. Nos. 4,411,494; 4,419,664; and 4,528,562; and F. J. Kahn (Appl. Phys. Lett., vol. 22, p. 111 (1973). These devices are based on the dielectric reorientation of the liquid crystals and response times are on the order of milliseconds.
Mixtures which exhibit a chiral smectic A mesophase are useful in a device as described by Lagerwall, et al. 1st International Symposium On Ferroelectric Liquid Crystals, Bordeaux-Arcachon, France, 1987. These mixtures exhibit an electrooptic effect which is termed a soft-mode ferroelectric effect and sub-microsecond switching can be achieved.
Devices employing materials with a smectic C mesophase are useful in device applications as described by Pelzl, et al. (Kristall Technik., vol. 14, p. 817 (1979); Mol. Cryst. Liq. Cryst., vol. 53, p. 167 (1979); Liquid Crystals, vol. 2, p. 21 (1987); and Liquid Crystals, vol. 2, p. 131 (1987)). These devices are based on the dielectric reorientation of the liquid crystals and the response times are slow.
A recent advance in the liquid crystal art has been the utilization of tilted chiral smectic liquid crystals, which are also termed ferroelectric liquid crystals, in devices which give microsecond switching and bistable operation not possible in any of the device applications described above. Ferroelectric liquid crystals were discovered by R. B. Meyer, et al. (J. Physique, vol. 36, pp. 1-69, 1975). A high speed optical switching phenomenon was discovered for the ferroelectric liquid crystals by N. A. Clark, et al. (Appl. Phys. Lett., vol. 36, p. 899 (1980) and U.S. Pat. No. 4,367,924).
Fluorine-containing ferroelectric liquid crystal materials have recently been developed. U.S. Pat. No. 4,886,619 (Janulis) discloses fluorine-containing chiral smectic liquid crystal compounds which comprise a fluorocarbon terminal portion and a chiral hydrocarbon terminal portion with the terminal portions being connected by a central core. U.S. Pat. No. 5,082,587 (Janulis) discloses achiral fluorine-containing liquid crystal compounds which comprise a fluorocarbon terminal portion and a hydrocarbon or another fluorocarbon terminal portion, the terminal portions being connected by a central core.
International Publication No. WO 91/00897 (Merck) discloses chiral or achiral ring compounds which may be used as components of chiral, tilted, smectic liquid-crystalline phases with ferroelectric properties. The compounds have the formula EQU R.sup.1 --A.sup.1 --A.sup.2 --Q--(CH.sub.2).sub.m --(CF.sub.2).sub.n --X
where R.sup.1 is an alkyl or perfluoroalkyl group with 1 to 12 carbon atoms, in which one or two non-adjacent CH.sub.2 or CF.sub.2 groups may be replaced by O-atoms, and/or --CO--, --COO--, --CH.dbd.CH--, --CH-halogen-, --CHCN--, --OCOCH-halogen-, or --COO--CHCN-- groups or where R.sup.1 is X--(CF.sub.2).sub.n --(CH.sub.2).sub.m --Q-- and X is H or F; A.sup.1 and A.sup.2 are mutually independently unsubstituted 1,4-phenylene groups or 1,4-phenylene groups substituted by one or two F-atoms, whereby one or two CH-groups may may be substituted by N; Q is --O--, --COO--, --OCO-- or a single bond; m is 1 to 10; and n is 2 to 8, with the proviso that m is 3 to 10 if Q is --COO-- or --OCO--.
The high speed switching of the ferroelectric liquid crystals can be utilized in many applications: light valves, displays, printer heads, and the like. In addition to the submicrosecond switching speeds, some ferroelectric device geometries exhibit bistable, threshold sensitive switching, making them candidates for matrix addressed devices containing a large number of elements for passive displays of graphic and pictorial information, as well as optical processing applications.